The present invention relates to transformers and, more specifically, to output transformers of the type for use with push-pull audio amplifiers.
The advantages gained by the use of a push-pull amplifier have been well known since almost the beginning of the use of vacuum tube power amplifiers. In a push-pull circuit one tube amplifies the positive half of the signal while the other tube amplifies the negative half of the input signal, and both halves are ultimately combined in the secondary of the output transformer. It is this output transformer that has been the principal limitation on the usefulness and applicability of the push-pull amplifier. More specifically, the transformer involves restricted bandwidth and introduces switching-transient related distortion. Thus, although the push-pull power amplifier may be constructed using conventional, simple components and an extremely simple circuit arrangement, the functional limitations of the output transformer have limited its usefulness.
The design goals which would make the output transformer the most efficient and most desirable are readily identifiable; however, the design approach to meeting these goals is generally considered to involve mutually exclusive elements. For example, it is known that for optimum efficiency in class B operation, each side of the push-pull circuit, which includes a transformer winding and an amplification device, such as a power transistor or a power vacuum tube, is alternately driven into the conductive state for approximately 180.degree. of the input signal. This has the effect that during a complete amplification cycle the current flow in each side of the push-pull circuit must switch on and off at the appropriate instant. Therein lies the problem in such transformer design, since this kind of switching is not an inherent characteristic of transformers.
Additionally, further problems are presented in the design of such output transformers by the fact that each side of the push-pull circuit constitutes a separate generator of opposite polarity. Each generator operates only during its "turned-on" portion of the cycle during the time the other side is turned off or open by means of the appropriate amplifying device in the push-pull amplification circuit. Thus, each side of the input portion of the transformer must be designed to match separately the load impedance. Therefore, it is necessary for the output transformer in a push-pull circuit to have a primary winding which has an end-to-end impedance of four times that of a single winding which might otherwise match the generator-to-load impedance. Looking at this another way, the load resistance will be the resistance seen when looking into one half of the primary of the output transformer, Thus, if each half of the total number of turns on the primary equals the number of turns on the secondary, the load resistance looking into half of the primary would be equal to that of the actual load which might be connected to the output. Nevertheless, since reflected resistance is proportional to the square of the turns ratio, the load resistance as seen across the total primary winding would be four times the actual load resistance.
This raises another problem presented, for example, by class B push-pull output transformers, in that it is important to maintain equal coupling between each half of the push-pull primary winding and the secondary or load winding over the entire frequency band of interest. Failure to accomplish equal coupling between such windings results in unbalanced loading of the output tubes or transistors of the class B push-pull amplifier, possibly causing damage to these components. In any event, such unequal coupling generates both phase and amplitude distortion in the output waveform.